The present invention relates to a method and apparatus for calibrating an electronic compass.
Electronic compasses have become rather ubiquitous these days. Today, many vehicles, including vehicles used by consumers, are supplied by their manufacturers with electronic compasses already installed. Additionally, electronic compasses certainly have military application and are used in military vehicles, including airborne, waterborne and terrestrial. Moreover, people on foot can use electronic compasses for recreational purposes, military purposes or other purposes. As such, electronic compasses have a wide range of applications.
An electronic compass is a device that indicates the yaw heading to an object by measuring the earth""s magnetic field. For example, if the compass is a simple hand held device, the compass may have a simple pointing device to direct it towards the object in question. The compass then reads out the heading of whatever direction it happens to be pointing. If the compass is built into a vehicle, for example, the compass may simply read out the direction the vehicle is headed. If the compass is part of an optical system, then it reads out the direction the optical system is pointed.
Since the Earth""s magnetic field is a weak field, and since metal objects distort that field, in general one must calibrate the output of an electronic compass to achieve reasonably accurate results. Classically, if the compass is a hand held or portable unit, this involves placing the compass on a mechanical device that rotates the compass to known orientations. Or, if the compass is disposed in a vehicle, this involves moving the vehicle (and its compass) in a circle. The compass output is recorded and compared against a known orientation, generating corrections to the compass output. A mechanical turntable can be used for this purpose when the compass is a hand held unit.
While this method is effective, it is laborious and slow, particularly if the electronic compass is meant to be accurate, since then more calibration data must be collected. The present invention provides a much faster, more automated solution to the problem of calibrating an electronic compass by using additional sensors. Instead of requiring 30 minutes with a mechanical turntable, the calibration operations can now be done in 2 minutes or less.
Brief and general terms the present invention provides a method and an apparatus for calibrating an electronic compass of the type having a compass sensor, a calibration table and a display. A computer operating under program control is preferably used to update the calibration table of the electronic compass in response to physical movement of at least the electronic compass sensor and an angular rate gyroscope through a 360 degree movement. A pointing device associated with the electronic compass is initially directed to a known landmark position and is also directed to the known landmark position after completing the 360 degree movement. The computer utilizes global positioning system receiver, the known landmark position and a heading provided by the sensor to generate a correction value. The output of the sensor is compared with data generated based upon the gyroscope during the 360 degree movement to generate correction values for other compass headings. The correction values are transmitted to the electronic compass for updating its calibration table.